(1) Field of the Invention
This invention relates to the casting of metals, particularly aluminum and aluminum alloys, by direct chill (DC) casting techniques. More particularly, the invention relates to the co-casting of metal layers by direct chill casting involving sequential solidification.
(2) Description of the Related Art
Metal ingots are commonly produced by direct chill casting of molten metals. This involves pouring a molten metal into a mold having cooled walls, an open upper end and (after start-up) an open lower end. Molten metal is introduced into the mold at the open upper end and is cooled and solidified (at least externally) as it passes through the mold. Solidified metal in the form of an ingot emerges from the open lower end of the mold and descends as the casting operation proceeds. In other cases, the casting takes place horizontally, but the procedure is essentially the same. Such casting techniques are particularly suited for the casting of aluminum and aluminum alloys, but may be employed for other metals too.
DC casting techniques of this kind are discussed extensively in U.S. Pat. No. 6,260,602 to Wagstaff, which relates exclusively to the casting of monolithic ingots, i.e. ingots made of the same metal throughout and cast as a single layer. Apparatus and methods for casting layered structures by sequential solidification techniques are disclosed in U.S. Patent Publication No. 2005/0011630 A1 to Anderson et al. Sequential solidification involves the casting of a first layer and then, subsequently but in the same casting operation; casting a layer of other metals on the first layer once it has achieved a suitable degree of solidification. Variations include casting outer layers of a multi-layer ingot first, and then casting a core layer within the outer layers once the outer layers have solidified suitably.
While these techniques are effective and successful, it has been found by the inventor of the present invention that difficulties may be encountered when attempting to employ the sequential solidification technique with certain combinations of alloys, particularly those having the same or very similar coefficients of contraction upon solidification and cooling. In particular, when such metals are sequentially cast, it has been found that the cladding layer may not bond as securely with the core layer as would be desired, particularly in the center region of the composite ingot.
There is therefore a need for improved casting equipment and techniques when co-casting metals of these kinds.